1. Field of the Invention
The present invention relates to an auto-focus apparatus which is improved to make an object lens movable based on a contrast of the object image so that the object lens can always focus on an object surface to be measured. The present invention is applicable to an optical measurement equipment such as an image measuring instrument, a microscope which carries out measurement on sizes and shapes of the object to be measured with observing an image of the object with an optical observation system.
2. Description of the Related Art
As the types of auto-focus apparatus provided on an optical measuring instrument such as an image measuring instrument or a microscope, there has been known a contrast method type to obtain focus based on the contrast of the image of an object surface to be measured. The image is first taken by a CCD camera. Or a laser focus method type to obtain focus based on the reflected light from the object surface to be measured using a laser.
But the above described contrast method or the laser method has following difficulties.
It is difficult for the former contrast method to bring correct focus on an essentially low contrast surface such as a mirror-finished surface or a glass surface.
In the latter laser focus method, it is possible to bring focus on the low contrast surfaces just described above but only on the spot radiated with the laser (most of the spots are on central portions). Moreover, the instrument itself is very expensive.
The object of the present invention is to provide an auto-focus apparatus which resolves the difficulties of conventional contrast method type or laser focus method type, enabling to focus on objects to be measured of all sort of materials, and on any optional spots within an observed image of the object to be measured.